1. Field of the Invention
The present invention relates to a positive displacement type liquid-delivery apparatus that can be used to deliver a very small amount of liquid at a constant rate to various processing apparatuses such as a chemical vapor deposition apparatus.
2. Description of the Related Arts
Recently, in the semiconductor manufacturing industry, the integration of integrated circuits has been improved remarkably, and the research and development activities of DRAM are being intensively carried out in anticipation of gigabit order DRAMs which will replace current megabit order DRAMs. A capacitor element having a large capacity per unit area is needed to produce such DRAMs. As a dielectric thin-film material for producing elements having such a large capacity per unit area, a metallic oxide film material such as tantalum pentaoxide (Ta2O5) having a dielectric constant of approximately 20, or barium titanate (BaTiO3) or strontium titanate (SrTiO3) or barium strontium titanate having a dielectric constant of approximately 300 is considered to be a promising thin-film material.
To deposit such a metallic oxide film material on a substrate in a vapor phase, a gaseous mixture made by mixing one or more gas feed materials of organometallic compounds and an oxygen containing gas is ejected to a substrate heated to a certain temperature. Organometallic gaseous feed material is chosen based on the nature of the thin film to be produced. For example, a metallic oxide film comprised by barium strontium titanate is produced by first converting Ba, Sr, Ti or their compounds into their dipivaloylmethane (DPM) compounds, and dissolving these compounds in an organic solvent such as tetrahydrofuran (THF) to produce respective liquid feed materials. After uniformly mixing these liquid feed materials in a required proportion to produce a master liquid feed, such master liquid feed is sent to a vaporizer to produce a gaseous feed for use in the chemical vapor deposition apparatus.
Such master liquid feed is extremely susceptible to degradation even in a sealed container, and therefore it is undesirable to have such a master liquid feed stagnate inside delivery piping. The master liquid feed is especially susceptible to producing precipitate particles, by being heated or being exposed to air, which tend to produce inferior quality films. Therefore, once the component liquids are mixed into a master liquid feed, it is necessary that the master liquid feed be maintained in a stable condition. It is also desirable that the master liquid feed be completely used up as quickly as practicable. Furthermore, it is desirable that the film deposition apparatus be capable of exercising a fine control of the flow rate of the master liquid feed over a wide range of flow rates from a very small flow rate to a large flow rate. Therefore, the liquid-delivering apparatus should be capable of providing a stringent control of the flow rates of the liquid feed.
As a positive displacement type liquid-delivering apparatus used in these applications, there has been known such an apparatus in which a mass flow controller (WFC) is provided in the piping connecting a feed liquid tank and a processing apparatus such as a vaporizer. The feed liquid tank is pressurized with gas or the like to deliver liquid, and a control valve on the MFC is adjusted to control a delivery rate of liquid. Positive displacement pumps incorporating pistons, diaphragms, and the like are also used.
In general, conventional apparatuses using a mass flow controller have a poor reproducibility of flow control near the lower limit of the allowable control range. Moreover, when the pressure in the processing apparatus increases, a pressure exceeding the pressure in the processing apparatus must be applied to the feed liquid tank side. Hence, a large amount of gas used for pressurizing is dissolved in the liquid in the feed liquid tank, and this dissolved gas is released downstream of the control valve of the mass flow controller or causes surge or pulsation in the flow of the liquid feed.
Although a positive displacement pump can overcome these drawbacks, a piston pump cannot be used because the sliding parts of the pump generate particles that contaminate the liquid. The positive displacement pumps employing bellows or diaphragms do not contaminate the liquid, but present the following problems.
It is conceivable to construct such a positive displacement pump in which a container is partitioned by a diaphragm into two chambers, i.e., a liquid delivery chamber and a working fluid chamber, and an incompressible liquid is used as a working fluid. With this construction, the diaphragm moves according to the amount of the working fluid supplied to the working fluid chamber for thereby discharging liquid from the container. Therefore, the precision in controlling the flow rate is more or less dependent on the precision of the external driving system. As a result, an external device is required for pumping the working fluid, and hence troublesome handling of the working fluid is necessary and the overall apparatus becomes large-sized.
If a driving device for driving the diaphragm is constructed mechanically, then these problems are eliminated and the overall apparatus becomes simple. However, it is very difficult to control the movement of the diaphragm so as to continuously deliver liquid at a constant rate if the processing conditions (pressure) in the secondary side (downstream side) of the container vary. Even if a flow meter is installed in the secondary side of the container for performing feedback control, it is not possible to obtain a better performance than that of the mass flow controller, because precision and reproducibility of the flow meter is the same level as the flow controller.
When the liquid-delivery is stopped, the pressure in the secondary side of the positive displacement pump slowly decreases due to a small leak in the check valve provided in the primary side (upstream side) of the processing apparatus (the part to which liquid is supplied). This may lead to a pressure drop when the liquid-delivery resumes, requiring time to stabilize the flow rate of liquid and potentially causing other problems. For example, if the pressure in the processing apparatus is below atmospheric pressure, the liquid feed may be vaporized because the pressure in the primary side of the check valve drops below the vapor pressure of the liquid feed.
Further, in the positive displacement pump, pressure variations occur in piping in the secondary side of the pump when the pumping operation begins, and hence the flow rate of liquid cannot be controlled until the liquid-delivery is stabilized. If a plurality of liquid feeds are required to be delivered at the same ratio, for example, these liquid feeds cannot be used until the liquid-delivery is stabilized.
In view of the foregoing, it is an object of the present invention to provide a positive displacement type liquid-delivery apparatus employing a positive displacement pump with a flexible diaphragm which can supply liquid at a constant rate with high precision and high reproducibility, shorten the time required to stabilize the liquid-delivery from the start of the pumping operation, and control the flow rate of liquid immediately after the pumping operation begins.
According to an aspect of the present invention, there is provided a positive displacement liquid-delivery apparatus comprising: a positive displacement pump comprising a housing having a liquid-delivery chamber divided by a flexible diaphragm and a diaphragm driving unit linked to the diaphragm to discharge fluid from the liquid-delivery chamber; and a differential pressure control unit for controlling the differential pressure between both sides of the diaphragm at a constant value during the pumping process.
Accordingly, the construction of the apparatus is simplified because the diaphragm is driven directly by the diaphragm driving unit. Further, by keeping the differential pressure between both sides of the diaphragm at a constant value, it is possible to keep the diaphragm at a constant amount of deformation, thus eliminating error caused by the diaphragm deformation. Hence, the diaphragm driving unit can control the amount of deformation in the diaphragm to perform precise flow rate control.
In a preferred aspect of the present invention, the differential pressure control unit comprises a differential pressure sensor for detecting the differential pressure between both sides of the diaphragm, and a control valve for controlling the flow rate of the liquid discharged from the liquid-delivery chamber on the basis of a signal from the differential pressure sensor.
Accordingly, it is possible to adjust the pressure in the liquid-delivery chamber indirectly by adjusting the control valve. If there is sufficiently low pressure variation in the space on the opposite side of the diaphragm from the liquid-delivery chamber, such as atmospheric pressure, the pressure sensor is required to be used only in the space on the side facing the liquid-delivery chamber.
In a preferred aspect of the present invention, a flow sensor is disposed on a discharge path and control is performed based on a signal from the flow sensor when the pressure in the liquid-delivery chamber during the pumping process exceeds a prescribed value or the absolute value of the rate of pressure variations exceeds a prescribed value.
With this construction, precise control can be preformed even with severe variations in the system conditions.
In a preferred aspect of the present invention, the liquid-delivery chamber is arranged so as to achieve the required discharge flow volume of the fluid in one stroke.
With this construction, the bellows operation is always stable and uniform for each process, thereby avoiding pressure and flow rate variations that occur, for example, when switching valves in alternate operations. Performing one pump operation using only a portion of one stroke can further increase the life of the bellows.
In a preferred aspect of the present invention, the gas is employed to pressurize the space on the opposite side of the diaphragm from the liquid-delivery chamber.
Generally speaking, the diaphragm itself has an allowable differential pressure between the sides of the bellows. When this differential pressure is small or the pressure required in the processing apparatus on the secondary side of the pump is larger than the allowable differential pressure, liquid-delivery cannot be performed if the pressure on the side of the diaphragm opposite from the liquid-delivery is atmospheric pressure. However, it is possible to keep the differential pressure low by pressurizing this side opposite the liquid-delivery chamber with a gas in order to maintain the differential pressure within the tolerable level for pumping operations.
Since the differential pressure of the diaphragm must be maintained at a constant value as described above in order to supply the fluid at a constant flow rate, the gas pressure P must also be constant. In the example described above, the volume V on the side of the diaphragm opposite the liquid-delivery chamber varies during pumping operations. Accordingly, the side of the diaphragm opposite the liquid-delivery chamber should be supplied with an amount of gas based on the liquid-delivery amount xcex94V, that is, xcex94Vxc3x97P.
The method of controlling the differential pressure between both sides of the diaphragm can be applied for using the pressure of the gas and the liquid, and controlling the pressure on the gas side. However, the injection and discharge of gas requires some time, resulting in control delays when pressure variations occur abruptly. Hence, variations in the differential pressure may occur more frequently, making it difficult to maintain a prescribed amount of liquid. Still, this method may be suitable for processes that have no severe pressure variations.
A leak sensor can be provided in the space opposite the liquid-delivery chamber for detecting fluid leaking caused by breakage in the diaphragm. With this arrangement, breakage in the diaphragm can be detected. If the side opposite the liquid-delivery chamber is also filled with liquid for driving the diaphragm, it is extremely difficult to detect breakage in the diaphragm. In the event that the diaphragm breaks, liquid for driving the diaphragm is mixed with the liquid to be pumped and the mixture is pumped together. Since the amount of liquid discharged from the apparatus does not vary, the breakage cannot be detected on a flow rate monitor.
In the present invention, however, breakage in the diaphragm can be detected by providing a relief discharge port, for example, on the gas side of the diaphragm and a relief sensor in the relief discharge port or on the secondary side. Further, it is possible to prevent gas from mixing with the pump side by always keeping the gas side at a lower pressure than the pump side. Hence, the present invention can avoid the problem of pumping liquid that mixes with driving liquid when the diaphragm breaks. Such problem is common to a conventional apparatus with fluid-driven diaphragms.
In a preferred aspect of the present invention, a plurality of positive displacement pumps are arranged in parallel and deliver different kinds of fluid to a single processing unit.
In a preferred aspect of the present invention, two positive displacement pumps deliver the same kind of fluid, and alternately deliver the fluid to a single processing unit in a continuous manner.
In a preferred aspect of the present invention, a housing having a liquid-delivery chamber is divided by a flexible diaphragm and a diaphragm driving unit linked to said diaphragm to discharge fluid from said liquid-delivery chamber. The diaphragm driving unit drives the diaphragm to maintain the flow rate of the liquid discharged from the liquid-delivery chamber at a constant rate based on the variation of the differential pressure between both sides of the diaphragm.
In a preferred aspect of the present invention, the liquid-delivery chamber is arranged so as to achieve the required discharge flow volume of the fluid in one stroke.
In a preferred aspect of the present invention, the gas is employed to pressurize the space on the opposite side of the diaphragm from the liquid-delivery chamber.
In a preferred aspect of the present invention, a plurality of positive displacement pumps are arranged in parallel and deliver different kinds of fluid to a single processing unit.
In a preferred aspect of the present invention, two positive displacement pumps deliver the same kind of fluid, and alternately deliver the fluid to a single processing unit in a continuous manner.
According to an aspect of the present invention, there is provided a positive displacement liquid-delivery apparatus comprising: a positive displacement pump comprising a housing having a liquid-delivery chamber divided by a flexible diaphragm and a diaphragm driving unit linked to the diaphragm to discharge fluid from the liquid-delivery chamber; a discharge path extending from the liquid-delivery chamber; a check valve disposed on the discharge path; and a pressure control unit for controlling the primary side pressure of the check valve so as not to drop below the vapor pressure of the fluid discharged from the liquid-delivery chamber during stoppage of the pumping process.
With this construction, it is possible to prevent a drop in pressure on the primary side of the check valve caused by a leak from the check valve and the generation of voids caused by vaporization.
In a preferred aspect of the present invention, the pressure control unit comprises a control valve disposed upstream of the check valve, and regulates the pressure in the liquid-delivery chamber during pump stoppage at the pressure required for pumping operation.
With this construction, if the pipe connecting the check valve and control valve is sufficiently short and formed of a highly rigid material and there is almost no volume expansion in this section of pipe when its internal pressure rises at the beginning of the pumping process, it is possible to set the pressure in the secondary side of the check valve to the normal pressure for pumping immediately after pumping begins in order to pump a prescribed flow rate without any time lag.
In a preferred aspect of the present invention, the pressure control unit comprises a control valve disposed upstream of the check valve, and regulates the pressure in the liquid-delivery chamber during pump stoppage at a pressure higher than the pressure required for pumping operation by an amount equivalent to the estimated amount caused by the volume expansion of the piping between the check valve and control valve.
With this construction, if this section of pipe is a flexible pipe with low rigidity and there is volume expansion in the pipe when the pressure rises at the beginning of the pumping process, it is possible to set the pressure in the secondary side of the check valve to the normal pressure for pumping immediately after pumping begins in order to pump a prescribed flow rate without any time lag.
In a preferred aspect of the present invention, the liquid-delivery chamber is arranged so as to achieve the required discharge flow volume of the fluid in one stroke.
In a preferred aspect of the present invention, the gas is employed to pressurize the space on the opposite side of the diaphragm from the liquid-delivery chamber.
In a preferred aspect of the present invention, a plurality of positive displacement pumps are arranged in parallel and deliver different kinds of fluid to a single processing unit.
With this construction, the apparatus can individually control a different flow rate of fluid discharged from each positive displacement pump from the moment the pumping process begins, thereby always pumping the same proportion of fluids to the single process device.
In a preferred aspect of the present invention, two positive displacement pumps deliver the same kind of fluid, and alternately deliver the fluid to a single processing unit in a continuous manner.
With this construction, it is possible to operate both pumps alternately such that the first pump gradually pumps a larger flow rate after the start of operations and the second pump gradually pumps a decreasing amount in order that the overall flow rate does not change. Accordingly, the same liquid can be supplied continuously to the single process device without variation in flow.
According to an aspect of the present invention, there is provided a deposition apparatus comprising: a vaporizer for vaporizing a fluid feed supplied from the positive displacement liquid-delivery apparatus; and a deposition chamber in which thin films are deposited using the feed gas supplied from the vaporizer.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.